1892
J. L. SETTLE,H. AI. FEDER, AND W. IY.HUBBARD
Vol. 67
FLUORINE BOMB CALORIMETRY. VI. THE ENTHALPY OF FORMATION OF URANIUM HEX4FLUORIDEl B Y JACK L.
SETTLE,
HAROLD &I. FEDER, AND
WrZRD
s.HUBBARD
Argcrnne Sataonal Laboratory, Argonne, Illinois Received M a y 1, 1963 The enthalpy of formation of uranium hexafluoride v a s measured by direct combination of the elements in a bomb calorimeter. Aefa2gs 16 (kcal. mole-’) = -522.64 =!= 0.43 (crystal) and -510.77 ==! 0.45 (gas).
Introduction This research is part of a continuing program to obtain thermochemical data by fluorine bomb calorimetry. As an essential preliminary to a study of various uranium-containing compounds a value for the enthalpy of formation of uranium hexafluoride more precise than existing estimates mas sought. It x a s found that under appropriate conditioiw the reaction between metallic uranium and fluorine gas yielded a product of which approximately 99% was uranium hexafluoride and only 1% mas lower fluorides. -4 method for the quantitative determination of the lower fluorides was developed in order t o make the required corrections. Experimental Calorimetric System.-The calorimeter, laboratory designation ANL-Rl , and the general combustion technique have been described.2 The combustion bomb, laboratory designation Xil M , will be described in a forthcoming publication in this series. The calorimetric measurements x ere made in the usual manner.a The energy equivalent, & (calor.), of the calorimetric svstem was determined by combustion in oxygen of benzoic acid (Sational Bureau of Standards Sample 39i) under prescribed conditions. A series of eight combustions, done half before and half after the calorimetric runs in fluorine, yielded a mean value of & (calor.) of 3G07.04 cal. deg.-’witha standard deviation of 0.26 ca1. deg.-’, or 0.007%. Materials.-Reactor grade uranium metal of normal isotopic composition was supplied in the form of two 3/32 X 3/32-in. rods and 0.005 and 0.01-in. thick foils. Table I is a summary of the impurities found in the ssmpIes. N o other metallic impurities were detected spectroscopically. 1-ranium comprieed 99.95 and 99.947, of the m i g h t of the tx-o rod samples; with certain assumptions (see below) as t o the state of combination of the impurities the samples contained 99.51 and 99.40 mole 7*metallic uranium. High-purity fluorine (99.99%) was prepared by distillation of commercial fluorine in a low temperature sti1L4 High-purity argon (>99.9yc) was obtained commercially. Procedure.-The uranium rods were machined to approximately 6/3-in. length pins. Both ends of each pin were turned down to in. diam. One end was slotted to accommodate a 0.20-g. piece of bent foil and a length of ignition wire weighing 0.07 g. The surface oxide on the pin, foil, and vire was removed by treatment with dilute nitric acid, follolxd by flushes with water and acetone. The parts were weighed, assembled, and placed in position in the bomb. The assemblj is shown in Fig. 1. [In order to minimize oxidation of the cleaned uranium parts, their exposure to air during assembly was kept a s short (about 15 min.) as possible. During this exposure no gain in weight was detected.] After the bomb was sealed evacuation was started immediately. The bomb was pumped doim to a pressure of
